1. Field of the Invention
The present invention relates to liquid crystal displays and, more particularly, to a vertically aligned pi-cell liquid crystal display with improved characteristics.
2. Description of the Related Art
Many technologies exist for making wide-viewing angle displays. Besides commonly used twisted nematic (TN) liquid crystal display (LCD) modes, there are other LCD modes, such as vertical alignment (VA) mode and pi-cell mode. Referring to FIG. 1A, a conventional VA mode is shown. The VA mode uses homeotropic (vertical) alignment material for alignment layer 10, anti-parallel rubbing with a small pretilt angle (liquid crystal molecules are tilted with a small angle away from the substrate normal direction)for alignment layer 10, negative dielectric anisotropic liquid crystal material 12 and a pair of cross polarizers 11 whose transmission axis makes 45.degree. with respect to the projection of the liquid crystal director onto a substrate 14 surface when the display panel is in the field-on state. A liquid crystal director is a local average of the liquid crystal molecules' direction. Because the homeotropic alignment material and the anti-parallel rubbing are used, in the field off state, the liquid crystal director is almost perpendicular to the substrate surface with a small pretilt angle, say 1.degree. to 10.degree., away from the substrate 14 normal. (The small pretilt is not shown in FIG. 1A.) This makes the liquid crystal material have virtually no effect on the incident light hitting substrate 14. Therefore, the display panel shows a dark state since cross polarizers 11 are used.
Referring to FIG. 1B, when a voltage is applied between electrodes 18 and 20, the liquid crystal molecules tend to tilt to the horizontal direction as shown, since a negative dielectric anisotropic liquid crystal material is used. Because the polarizers 11 are arranged in a manner such that the transmission axis makes a 45.degree. angle with respect to the projection of the liquid crystal director, in the field-on state, the polarization of light is changed by the liquid crystal medium and light transmits through the display panel. Thus, a bright state is provided.
Referring to FIG. 2A, a conventional pi-cell is shown. The pi-cell mode uses homogenous (planar) alignment material for alignment layer 30, parallel rubbing with a proper pretilt angle (liquid crystal molecules are tilted with a small angle away from the substrate surface) for alignment layer 30, positive dielectric anisotropic liquid crystal material 32 and a pair of cross polarizers 31 whose transmission axis makes 45.degree. with respect to the projection of the liquid crystal director onto a substrate 34 surface when the display panel is in the field-on state. A pi-cell can be operated in a normally black (NB) mode, in which display is a dark state when a low voltage is applied to electrodes 36 and 38, or a normally white (NW) mode, in which display is a bright state when a low voltage is applied. For illustration purposes, a NB is explained below. (The difference between NB mode and NW mode of a pi-cell comes from the difference in birefringence (d*(n.sub.e -n.sub.o)) of the panel (where d is thickness of cell gap (between alignment plates), n.sub.e and n.sub.o are the extraordinary and ordinary of the indices of refraction of the liquid crystal material.) Because a homogenous alignment material and a parallel rubbing with a proper pretilt angle are used, in the low-field state, the liquid crystal director is arranged in a bend state as shown in FIG. 2A. The birefringence of the panel is adjusted such that d*(n.sub.e -n.sub.o)=.lambda. (where d is thickness of the cell gap, n.sub.e and n.sub.o are the extraordinary and ordinary of the indices of refraction of the liquid crystal material, respectively, .lambda. is the wavelength of the incident light in the air). In the low-field state the panel is dark. When a high voltage is applied (high-field state) across electrodes 36 and 38, the liquid crystal director configuration is rearranged so that liquid crystal molecules are more perpendicular to the substrate surface since a positive dielectric anisotropic liquid crystal material is used. Thus, the birefringence of the panel changes and light is transmitted through the panel and a bright state is achieved.
Both the conventional TN and the conventional VA liquid crystal displays suffer from narrow viewing angles. Since the conventional pi-cell is operated in a birefringent mode, the pi-cell does not produce a good dark state.
Therefore, a need exists for a new liquid crystal mode which provides wide viewing angle, fast response speed, high brightness, a good dark state and a high contrast.